1. Field of the Invention
The present invention relates generally to an antenna apparatus and wireless communication terminal, and more particularly, to an antenna apparatus that can control directivity of a plurality of radiation elements using a parasitic element.
2. Description of the Related Art
Recently, communication technologies for realizing high-speed communication in portable terminals, such as hand-held phones, PDAs, etc., have been in the spotlight. Some of them (for example, Multiple Input Multiple Output (MIMO)) have been put to practical use, in which both a transmitting device and a receiving device have a plurality of antennas and transmit/receive signals through the antennas, respectively. Such technologies enable very high-speed communication because information corresponding to the number of a plurality of antennas can be received, though there is overhead in a signal separation process, etc. However, in the application of a MIMO system, it is necessary to mount a plurality of antennas with suitable directivity to a small-sized box body of the portable terminal. For this reason, it has been required to manufacture small-sized antennas with desired directivity. More specifically, a device capable of simply controlling directivity of a plurality of antennas has been required.
As the control technique of the antenna directivity, a manner where driven elements are disposed in the front side and rear side of a radiation element is well-known in the art. A typical example of this is a Yagi-Uda antenna. The Yagi-Uda antenna has already been put to practical use and is now in wide use as a receiving antenna for an analog TV. As an application of the Yagi-Uda antenna, Japanese Patent No. 2005-210521 discloses a planar plate antenna that includes a slot yagi antenna and driven elements disposed round about the slot yagi antenna, wherein electric fields are converted to control directivity. As for a resonant frequency, the driven elements of this antenna can be configured to function as reflectors by forming them to be relatively long and as directors by forming them to be relatively short. Therefore, this antenna can convert the directivity according to the lengths of the driven elements.
However, in order to permit vertically polarized radiation, the antenna described above should be enlarged in size in a horizontal direction due to a structural characteristic thereof. Thus, this antenna is difficult to mount onto portable terminals. Also, the user usually holds the portable terminal slightly tilted in front of him when using it. Therefore, when the antenna is mounted to the portable terminal, directional peaks go toward the user's body or the ground blocking them, so it is difficult for this antenna to obtain sufficient communication quality ensuring high-speed communication in a MIMO system.
In another example, a monopole type antenna is mounted at a front end of a portable terminal. This antenna also does not provide satisfactory communication quality since directional peaks of radio waves radiated from the antenna go toward the ground. Contrarily, it can also be considered that a monopole type antenna is mounted to the portable terminal in the direction of the rear end of the terminal, but directional peaks of radio waves affected by a base board go toward the user's body to block them, whereby the sensitivity of the radio waves is reduced.